The present invention relates to an internal combustion engine including a cylinder block and a piston. The cylinder block includes at least one cylinder bore, and the piston is located in the cylinder bore. The piston includes at least one annular groove, and a piston ring is inserted into the annular groove of the piston.
More particularly, the present invention relates to improvements in the piston ring and the annular groove of the piston for preventing a top portion of the piston from being melted by heat in a combustion chamber.
Japanese Utility Model Laid Open Application No. 57-49536 discloses a conventional engine with a cylinder bore in a cylinder block and a piston. A top piston ring is inserted into a top annular groove of the piston, and the piston with the top piston ring is located in the cylinder bore. The top piston ring has an "L" cross-sectional shape (i.e., the top piston ring includes an outer peripheral portion and an inner peripheral portion). The outer peripheral portion extends upwardly in the longitudinal direction of the cylinder bore, and the inner peripheral portion extends in the lateral direction of the cylinder bore. A top portion of the outer peripheral portion of the top piston ring is located at substantially the same level as a top surface of the piston. The outer peripheral portion of the top piston ring surrounds a top land of the piston.
Accordingly, heat in a combustion chamber is transmitted to the piston and then to the cylinder block through the top piston ring. As a result, the top piston ring prevents the top land of the piston from being melted by heat in the combustion chamber. Nonetheless, this arrangement suffers from a substantial disadvantage.
When the piston is displaced upwardly and air is compressed in the combustion chamber, the compressed air pushes against the top piston ring. The inner peripheral portion of the top piston ring is not displaced downwardly because the lower surface of the top annular groove of the piston extends perpendicular to the longitudinal direction of the cylinder bore, and the inner peripheral portion of the top piston ring is pushed against the lower surface of the top annular groove. However, the outer peripheral portion of the top piston ring is pushed against a cylinder bore surface of the cylinder bore by the compressed air in the combustion chamber (i.e., the outer peripheral portion of the top piston ring is pushed into tight contact with the cylinder bore surface). As a result, when the piston is displaced along the longitudinal direction of the cylinder bore, excessive friction is caused between the outer peripheral portion of the top piston ring and the cylinder bore surface of the cylinder bore. Energy of the internal combustion engine is necessarily expended to overcome this friction.